利用理論計算探討含醯胺官能基之金屬有機框架材料的二氧化碳捕捉性質

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(1)國立臺灣師範大學化學系碩士論文. 利用理論計算探討含醯胺官能基之金屬有機框架材料的二氧化碳捕捉性質 Computational Study on Carbon Capture of Amide-Containing Metal− −Organic Frameworks. 指導教授：指導教授：趙奕姼博士. 研究生：研究生：黃鴻裕撰. 中華民國一百零二年八月.

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(5) 摘要瞭解二氧化碳與吸附劑在微觀尺度下的交互作用機制將有助於設計出更為適合捕捉二氧化碳的材料。本篇論文探討了兩個含醯胺官能基之金屬有機框架材料的碳捕捉性質。兩個材料分別是 {[Zn4(bdc)4(bpda)4]·5DMF·3H2O}n ( 化合物 1 ， bdc 為 1,4-benzyl dicarboxylate，bpda 為 N,N′-bis(4-pyridinyl)-1,4-benzenedicarboxamide) 與{[Cd3(4-btapa)2(2,6-ndc)3]·9H2O}n (化合物 2，4-btapa 為 1,3,5-benzene tricarboxylic acid tris-N-(4-pyridyl)amide，2,6-nbc 為 2,6-naphthalene dicarboxylate)。在化合物 1 與 2 的結構裡，可見到未受遮蔽之醯胺官能基在孔洞中的空間佈置情形特別密集，這對於兩者吸附二氧化碳的能力有何影響，為本篇論文的探討重點。論文中利用維里展開式擬合法推估兩者的二氧化碳等量吸附熱與變化趨勢，瞭解孔洞表面與二氧化碳之間的親合程度。以及借助密度泛函理論計算推測吸附二氧化碳的吸附模式並進行結合能分析，試圖解釋等量吸附熱的變化情形，瞭解醯胺官能基的空間佈置對材料碳捕捉性質的影響。此外，根據理想吸附溶液理論預測兩者對 CO2/N2 混合氣體之吸附選擇性。理論計算的結果顯示化合物 1 主孔道立壁的牆面，因為有著未受遮蔽之醯胺官能基櫛比鱗次地正反相疊的緣故，而有著極性正反相間的帶狀區。這些帶狀區不僅能夠提供二氧化碳與醯胺官能基發生正向協同性結合 i.

(6) 的吸附位置，還可營造出促使二氧化碳彼此交互作用環境，讓兩個二氧化碳分子以錯開平行的方式排列。這些發現有助於解釋為何化合物 1 吸附二氧化碳時的等量吸附熱會隨著吸附量增加而提高。而據理想吸附溶液理論的預測，化合物 1 的 CO2/N2 吸附選擇性，在二氧化碳與氮氣的混和比為 15:85 與溫度在 298 K 時，從低壓到室壓的數值為 24 至 23。化合物 2 的二氧化碳等量吸附熱，目前在含醯胺官能基之金屬有機框架材料當中為最高。理論計算結果呈現化合物 2 的結構中，兩個彼此正對相疊之 4-btapa 有機配位子的醯胺官能基能營造出獨特的吸附位置，可使二氧化碳受到醯胺官能基環繞而得以享有多點交互作用。化合物 2 於 298 K 的 15/85 CO2/N2 吸附選擇性，經理想溶液吸附理論的預測可知從低壓至室壓的數值為 28 至 24。關鍵字：醯胺官能基、金屬有機框架材料、二氧化碳、正向協同性結合效應、多點交互作用. ii.

(7) Abstract Understanding the interaction between CO2 and sorbent at the atomic level is the key to designing suitable systems for carbon capture and storage (CCS). In this work, the carbon dioxide adsorption properties of two amide-containing metal-organic frameworks (MOFs) are studied. Two MOFs are formulated as {[Zn4(bdc)4(bpda)4]·5DMF·3H2O}n (1, bdc = 1,4-benzyl dicarboxylate, bpda = N,N′-bis(4-pyridinyl)1,4-benzenedicarboxamide) and {[Cd3(4-btapa)2(2,6-ndc)3]·9H2O}n (2, 4-btapa = 1,3,5-benzene tricarboxylic acid tris-N-(4-pyridyl)amide, 2,6-nbc = 2,6-naphthalene dicarboxylate), respectively. A tight arrangement of unsheltered amide groups inside the pore of 1 and 2 is observed. This thesis puts the emphasis on the influence of spatial arrangement of amide groups upon carbon capture of these materials. The isosteric heats of CO2 adsorption (Qst) are determined by Virial method. The magnitude of Qst dictates the affinity of the pore surface toward CO2. Density functional theory (DFT) calculations were performed to gain insight about the CO2 adsorption mode in 1 and 2. The result of binding energy analysis for various adsorption modes attempt to interpret the trend of Qst. 15/85 CO2/N2 adsorption selectivities (Sads) are predicted by ideal adsorbed solution theory (IAST) in addition. The computational results showed that the unsheltered amide groups in the major channel of 1 form a belt of alternating polarity. Furthermore, we found the amide belts offered an environment for cooperative binding not only for CO2·amide attraction, but also for dimeric CO2 interaction in a slipped-parallel geometry. These findings help to explain the observed increase of Qst from low to 1:1 CO2:unsheltered amide adsorption ratio for 1. The Sads for 15/85 CO2/N2 mixtures of 1 at 298 K from low pressure to ambient pressure is from 24 to 23. Compound 2 has a high CO2 Qst value, which so far ranks top one among the amide-containing MOFs. The computational results showed that the amide groups of 4-btapa ligands form unique adsorption sites for carbon dioxide, in which a CO2 molecule is surrounded by amide groups with multipoint interactions between two 4-btapa ligands. IAST-predicted adsorption selectivity for 15/85 CO2/N2 mixtures of 2 at 298 K from low pressure to ambient pressure is from 28 to 24. Keyword: amide, MOFs, carbon dioxide, cooperative binding, multipoint interaction iii.

(8) 誌謝在攻讀碩士學位的期間，讓我見到了許多我所不知道的自己，更是受到不少的幫助，心中對此滿懷感激。首應感謝指導教授─趙奕姼老師。若是少了老師的悉心指導，肯定無法完成論文。非常謝謝老師提供開放的研究環境，讓我能嘗試感興趣的問題。整個過程中，帶領著我在彼此都不熟悉的領域裡一同探索與學習。並在研究與處事上不斷給予期勉與鞭策，著實獲益良多。感謝口試委員─呂光烈老師與蔡明剛老師在百忙之中撥冗給予指導與建議，讓此篇論文能夠更加完善，在此表達誠摯謝忱。特別謝謝合作夥伴─政樺學長總是能空出時間與我一起討論許多實驗上的結果，並從不同角度提出許多看法，從中獲悉不少知識與眉角，使我更加深刻了解實驗與計算結果彼此之間的連結所在，進而可將兩者併整統合。在研究所的這段時間，感謝實驗室成員的諸多幫助。當計算上碰到阻礙，感謝盧博士秀鳳的經驗傳授與提醒，因此可以豁然開朗。如果沒有偉韜學長的幫助，那麼我在計算實驗新手村駐足的時間將會拉長不少。英文方面，感謝李博士雲嫦給予意見與幫忙訂正。在為一些數學問題而困擾不已時，幸好有君豪學長指點迷津。在與威智學長討論、談天的過程當中，總能感到來自學長的關心與打氣。在美編、注意事項與許多小細節上，多虧了蕭竣學長專業的提點。謝謝宜容學姐為我的簡報提出許多很棒的建議。謝謝 iv.

(9) 韋豪學長的有機教學與音樂推薦。另外，也不會忘記張伯伯的「呼吸」任務。再次感謝實驗室夥伴的照顧與關懷。除此之外，鈞智、富傑、雅筑、貴緯與人豪，這段時間有你們情義相挺，真好。最後感謝能夠理解並支持我的家人以及我所不知道的自己。謝謝。. v.

(10) 目錄中文摘要......................................................................................................................... i 英文摘要...................................................................................................................... iii 誌謝.............................................................................................................................. iv 目錄.............................................................................................................................. vi 圖目錄........................................................................................................................ viii 表目錄......................................................................................................................... xv 第一章研究背景與論文回顧...................................................................................... 1 1-1 溫室效應 ........................................................................................................ 2 1-2 全球暖化及相關科學證據 ............................................................................ 2 1-3 碳捕捉與封存 ................................................................................................ 9 1-3-1 燃燒後端捕捉途徑 ............................................................................ 12 1-3-2 燃燒前端捕捉途徑 ............................................................................ 14 1-3-3 富氧燃燒後端捕捉途徑 .................................................................... 16 1-3-4 碳捕捉技術與碳捕捉材料 ................................................................ 17 1-4 金屬有機框架材料 ...................................................................................... 22 1-5 提升材料吸附二氧化碳能力的策略 .......................................................... 28 1-5-1 材料含有配位未飽和之金屬位點 .................................................... 29 1-5-2 將高極性官能基團引入材料之中 .................................................... 31 1-6 介紹研究案例─化合物 1 與化合物 2 ....................................................... 39 1-6-1 化合物合成簡介 ................................................................................ 39 1-6-2 化合物結構描述 ................................................................................ 40 第二章計算方法........................................................................................................ 45 2-1 理論計算部分 ............................................................................................... 45 2-2 等量吸附熱 ................................................................................................... 49 2-2-1 等量吸附線法 .................................................................................... 51 vi.

(11) 2-2-2 等溫吸附模型擬合法 ........................................................................ 51 2-2-3 維里展開式擬合法 ............................................................................ 52 2-2-4 Multi-site Langmuir 擬合法 .............................................................. 54 2-3 吸附選擇性 ................................................................................................... 56 2-3-1 吸附量轉換 ........................................................................................ 56 2-3-2 擬合等溫吸附曲線 ............................................................................ 59 2-3-3 理想吸附溶液理論 ............................................................................ 62 第三章結果與討論.................................................................................................... 66 3-1 化合物 1 的二氧化碳捕捉性質 ................................................................... 66 3-2 結論 ............................................................................................................... 87 3-3 化合物 2 的二氧化碳捕捉性質 ................................................................... 88 3-4 結論 ............................................................................................................. 100 參考文獻.................................................................................................................... 101 附錄............................................................................................................................ 114. vii.

(12) 圖目錄 Figure 1-1.. Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). The net anthropogenic radiative forcing and its range are also shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness. .............................. 5. Figure 1-2.. Observed changes in (a) global average surface temperature, (b) global average sea level from tide gauge (blue) and satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for the period 1961–1990. Smoothed curves represent decadal average values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c). ............ 7. Figure 1-3.. Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models using natural. and. anthropogenic forcings.. Decadal. averages. of. observations are shown for the period 1906 to 2005 (black line) plotted against the centre of the decade and relative to the corresponding average for 1901–1950. Lines are dashed where spatial coverage is less than 50%. Blue shaded bands show the 5–95% range for 19 simulations from five climate models using only the natural forcings due to solar activity and volcanoes. Red viii.

(13) shaded bands show the 5–95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. ...... 8 Figure 1-4.. Global greenhouse gas emission sources in 2004 of which approximately 77% are represented by CO2 emissions. ................ 10. Figure 1-5.. Three options for CO2 capture from power generation plants. ...... 11. Figure 1-6.. Carbon capture technologies include solvents, solid sorbents, membranes, and cryogenic distillation........................................... 18. Figure 1-7.. Different carbon capture technologies and associated materials for CO2 separation and capture. ........................................................... 18. Figure 1-8.. Schematic diagrams of idealized temperature swing adsorption (TSA), pressure swing adsorption (PSA), and vacuum swing adsorption (VSA) processes for regenerating solid adsorbent in a fixed-bed column. .......................................................................... 20. Figure 1-9.. A portion of the crystal structure of Zn4O(BDC)3 (MOF-5). Blue tetrahedra represent ZnO4 units, while gray and red spheres represent C and O atoms, respectively; H atoms are omitted for clarity.............................................................................................. 23. Figure 1-10.. Components of coordination polymers. ......................................... 23. Figure 1-11.. Different types of second building units (SBUs). .......................... 24. Figure 1-12.. Examples of SBUs from carboxylate MOFs. Color scheme: O, red; N, green; C, black. In inorganic units, metal-oxygen polyhedra are blue, and the polygon or polyhedron defined by carboxylate carbon atoms (SBUs) are red. In organic SBUs, the polygons or polyhedrons to which linkers (all –C6H4– units in these examples) are attached are shown in green. .................................................... 24. Figure 1-13.. (a) Twelve of the binodal edge-transitive nets in augmented form. (b) Twelve more binodal edge-transitive nets in augmeted form. . 25. Figure 1-14.. (a) the edge-transitive 2-periodic nets. (b) the augmented version. ........................................................................................................ 25. Figure 1-15.. Crystal structures of MOFs examined for CO2 storage capacity at ix.

(14) room temperature. For each MOF, the framework formula, pore size, and surface area are given. ..................................................... 27 Figure 1-16.. Comparison of gravimetric CO2 capacities for several MOFs (and an activated carbon as a reference) determined at ambient temperature and pressures up to 42 bar. ......................................... 27. Figure 1-17.. Bottom-up strategies to tune CO2 capture performance. The representative MOFs are from references[39] ............................... 29. Figure 1-18.. (a) View of 1D channels present in the M/DOBDC structure (solvent omitted). (b) Space-filling model of the pore structure of 1 unit cell of Mg/DOBDC (Mg atoms, green; C atoms, gray; O atoms, red; H atoms and solvent molecules removed for clarity) occupied by 12 molecules of CO2 (blue) which represents the sorption at 0.1 atm and 296 K. ....................................................... 31. Figure 1-19.. (Left) (a) Structure of the Zn-Atz layer in Zn2(Atz)2(ox). (b) Three-dimensional structure of Zn2(Atz)2(ox), wherein the Zn-Atz layers are pillared by oxalate moieties to form a six-connected cubic network shown as green struts. (c) Adsorption isotherms for different gases carried out using Zn2(Atz)2(ox). (Right) X-ray structure of CO2 binding in Zn2(Atz)2(ox)·(CO2)1.3 at 173 K. (a) The role of the amine group of Atz in binding CO2-I is depicted. The H atoms of the amine group (located crystallographically) H-bond to oxalate O atoms, directing the N lone pair toward the C(δ+) atom of the CO2 molecule. H-bond distances shown are for H-acceptor interactions. (b) Both crystallographically independent CO2 molecules are shown trapped in a pore, showing the cooperative interaction between CO2-I and CO2-II molecules. The CO2…NH2 interaction is represented as a dotted purple bond, and the CO2…CO2 interaction is indicated as a dotted yellow bond. (c) This panel shows the other interactions present. The CO2-I…Ox interactions are shown in orange, and the CO2…NH2 hydrogen bond interactions are shown in green. For clarity, H atoms are shown in purple. ............................................................................. 34 x.

(15) Figure 1-20.. (a) Functionalization of the metal−organic framework Cu-BTTri through binding N,N'-dimethylethylenediamine (mmen) to the open metal coordination sites. The basic amine groups dangling within the framework pores lead to strong CO2 adsorption, with a zero-coverage isosteric heat of adsorption of -96 kJ/mol. (b) CO2 (■) and N2 (●) adsorption isotherms collected at 298 K for mmen-Cu-BTTri (green symbols) and Cu-BTTri (blue symbols). (c) a plot of the isosteric heat of adsorption of CO2 (■) and N2 (●) in mmen-Cu-BTTri (green symbols) and Cu-BTTri (blue symbols). ........................................................................................................ 35. Figure 1-21.. (a). N,N',N''-tris(isophthalyl)-1,3,5-benzenetricarboxamide. (TPBTM). and. 5,5′,5′′-. benzene-1,3,5-triyltris(1-ethynyl-2-isophthalate) (btei). (b) Portion of the structure of the (3, 24)-connected rht-type framework of [Cu24(TPBTM6-)8(H2O)24](1)showing. surface. decoration. by. acylamide groups. (c) High-pressure gravimetric excess CO2 and N2 sorption isotherms of [Cu24(TPBTM6-)8(H2O)24](1)and the PCN-6X series at 298 K. Lines and symbols represent adsorption and desorption, respectively. (d) Isosteric heats of CO2 adsorption for [Cu24(TPBTM6-)8(H2O)24](1) and PCN-61............................... 38 Figure 1-22.. Structures of 1: (a) Side view of an α-Polonium-type cage. (b) Simplified view of a six-connected node. (c) The two-fold interpenetrated 412·63 net................................................................ 41. Figure 1-23.. (a) Net-to-net N−H…O hydrogen bonding interactions of in 1. (b) Superimposed space-filling representation of 1 showing two different sized channel. (c) A spotlight of the larger channel opening showing a nearly unique arrangement of the unsheltered amide-groups in 1. .......................................................................... 42. Figure 1-24.. (a) The coordination environment of Cd(II) dimer in 2. (b) View along the c axis, reveals that the 3D self-penetrating structure of 2. (c) View of the self-penetrating topology, based on (6,3) layer to (3,4)-connected network with (63)2(64·8·10)3 topology futher xi.

(16) self-penetrating to (3,6)-connected network with (63)2(614·8)3 topology. ........................................................................................ 44 Figure 2-1.. (A–C) Schematic representation of the relation between the surface excess mass and absolute adsorbed amount. Dark gray regions in the top diagrams represent the surface excess mass. Since absolute adsorbed amounts should include all gas molecules (dark gray and white circles in the bottom boxes) in the adsorbed layer, summation of the dark gray and light gray areas in the diagrams corresponds to the absolute adsorbed amount. The density of excess mass is not necessarily higher than the bulk fluid density at all locations, and negative excess mass can be obtained under large bulk density. (D) The surface excess amount (solid curve) possesses a maximum value because the excess mass can be decreased in the high-pressure region, while total gas uptake (dashed curve) should increase monotonically with an increase in the pressure because of the contribution from the large bulk density (dotted curve, rbulk) of the adsorbate. ........................................... 58. Figure 3-1.. CO2 adsorption isotherms measured at 273 K (black solid circle) and 298 K (red solid square) for 1 , respectively. .......................... 68. Figure 3-2.. CO2 adsorption isotherms of 1 fitted using the virial equation (green lines). ................................................................................... 69. Figure 3-3.. CO2 isosteric heat of adsorption (Qst) of 1. .................................... 69. Figure 3-4.. (a). The. atomic. structure. of. a. fragment. of. 1 and its electrostatic potential map. (b) The atomic structure of 1-a and its electrostatic potential map. (In ball-and-stick style; C = gray; H = white; O = red; N = blue; Zn = green) The exposed amide groups are marked by red dashed circles. The colors of the electrostatic potential map, ranging from red to blue, correspond to electrostatic potential ranging from −269.989 to 318.282 kJ mol-1. Red, green, and blue represent negative, zero, and positive values, respectively. ................................................................................... 71. xii.

(17) Figure 3-5.. (a) Side view of the M06-2X/6-31G** optimized structure of the CO2·1-a shown with a ball-and-stick representation and (b) geometry parameters labeled on the top view of the same complex shown in the wireframe representation, except that the amide groups and CO2 are in the ball-and-stick style. (Distances are in angstrom, angles in degree.)........................................................... 73. Figure 3-6.. Top view of the optimized structure of the 2CO2·1-b complex calculated at the M06-2X/6-31G** level shown in the wireframe representation, except that the amide groups and CO2 are in the ball-and-stick style. ........................................................................ 77. Figure 3-7.. Top view of the optimized structure of the 2CO2·1-c complex calculated at the M06-2X/6-31G** level shown in the wireframe representation, except that the amide groups and CO2 are in the ball-and-stick style. ........................................................................ 79. Figure 3-8.. Top view of the optimized structure of the 2CO2·1-d complex calculated at the M06-2X/6-31G** level shown in the wireframe representation, except that the amide groups and CO2 are in the ball-and-stick style. ........................................................................ 80. Figure 3-9.. Top view of the optimized structure of the 2CO2·1-e complex calculated at the M06-2X/6-31G** level shown in the wireframe representation, except that the amide groups and CO2 are in the ball-and-stick style. ........................................................................ 81. Figure 3-10.. The absolute loadings for CO2 in 1 at 273 K (black hexagon), and 298 K (red square). The green lines are dual-site Langmuir (DSL) fits................................................................................................... 85. Figure 3-11.. The absolute (navy pentangle) and excess (blue inverted triangle) loading for N2 in 1 at 298 K. The green line is single-site Langmuir (SSL) fit. ......................................................................................... 85. Figure 3-12.. IAST-predicted adsorption selectivity for 15/85 CO2/N2 mixtures of 1 at 298 K. .................................................................................. 86. Figure 3-13.. IAST-predicted isotherm for 15/85 CO2/N2 mixtures of 1 at 298 K xiii.

(18) as a function of total bulk pressure. ............................................... 87 Figure 3-14.. CO2 adsorption isotherms of 2 fitted with the use of the virial equation (green lines). .................................................................... 89. Figure 3-15.. CO2 isosteric heat of adsorption (Qst) of 2. .................................... 89. Figure 3-16.. Illustration of multipoint bonded CO2 with NC=O…CO2 and N-H…O=C=O interactions between two 4-btapa ligands. ............. 91. Figure 3-17.. Side view of the M06-2X/6-31G** optimized structure of the 3CO2·2-a shown with a ball-and-stick representation, except that carbon dioxide molecules are in the spacefilling style................... 91. Figure 3-18.. (a) The M06-2X/6-31G** optimized structure of 3CO2·2-a complex (Look from the side containing CO2(1)) shown with a spacefilling representation, except that CO2(1) is in the ball-and-stick style and (b) geometry parameters labeled on the same complex shown in the ball-and-stick syle. ............................ 92. Figure 3-19.. The electrostatic potential map of (a) 2-a and (b) carbon dioxide molecule. The colors of the electrostatic potential map, ranging from red to blue, correspond to electrostatic potential ranging from −269.989 to 318.282 kJ/mol. Red, green, and blue represent negative, zero, and positive values, respectively. .......................... 93. Figure 3-20.. The absolute loadings for CO2 in 2 at 273 K (dark gray circle), and 298 K (pink triangle). The green lines are dual-site Langmuir (DSL) fits. ...................................................................................... 97. Figure 3-21.. The absolute (green hexagon) and excess (blue diamond) loading for N2 in 2 at 298 K. The green line is single-site Langmuir (SSL) fit. ................................................................................................... 98. Figure 3-22.. IAST-predicted adsorption selectivity for 15/85 CO2/N2 mixtures of 2 at 298 K. .................................................................................. 99. Figure 3-23.. IAST-predicted isotherm for 15/85 CO2/N2 mixtures of 2 at 298 K as a function of total bulk pressure. ............................................... 99. xiv.

(19) 表目錄 Table 1-1.. Typical Post-combusion Flue Gas Composition for a Coal-Fired Power Plant. .......................................................................................13. Table 1-2.. Composition of Shifted Product of Coal Gasification. ......................15. Table 1-3.. Physical Parameters of Gases Relevant to Carbon Dioxide Capture Processes............................................................................................16. Table 1-4.. The differences between. chemical. adsorption and physical. adsorption. .........................................................................................20 Table 1-5.. 化學吸收法、吸附法與薄膜分離法優缺點比較與應用途徑 .......21. Table 2-1.. The BSSE corrected binding energy (Ecorr) and BSSE of CO2·formamide complex at the M06-2X, ωB97X-D, B97-D, B3LYP and MP2 levels using various basis sets. (kJ/mol) ............................47. Table 3-1.. The BSSE-corrected binding energy analysis for various complexes of CO2·1-a. ........................................................................................74. Table 3-2.. The BSSE-corrected binding energy analysis for various complexes of 2CO2·1-b. ......................................................................................77. Table 3-3.. The BSSE-corrected binding energy analysis for various complexes of 2CO2·1-c. ......................................................................................79. Table 3-4.. The BSSE-corrected binding energy analysis for various complexes of 2CO2·1-d. ......................................................................................80. Table 3-5.. The BSSE-corrected binding energy analysis for various complexes of 2CO2·1-e. ......................................................................................81. Table 3-6.. The fitted parameters of the dual-site (for CO2) or single-site (for N2) Langmuir model for the pure component isotherms in 1. .................86. Table 3-7.. Zero-coverage Qst value of CO2 adsorption of physical sorption in MOFs. ................................................................................................90. Table 3-8.. Zero-coverage Qst value of CO2 adsorption of physical sorption in amide-containing MOFs. ...................................................................90 xv.

(20) Table 3-9.. The BSSE-corrected binding energy analysis for various complexes of 3CO2·2-a. ......................................................................................94. Table 3-10.. The fitted parameters of the dual-site (for CO2) or single-site (for N2) Langmuir model for the pure component isotherms in 2. .................98. xvi.

(21) 第一章研究背景與論文回顧本篇論文為利用理論計算探討含醯胺官能基之金屬有機框架材料(metal−organic frameworks, MOFs)的二氧化碳捕捉性質之研究。本章為研究背景與論文回顧，首先闡述關注材料捕捉二氧化碳性質的緣起─全球暖化(global warming)，將談及何謂溫室效應(greenhouse effect)以及此效應與全球暖化的關係，並引用聯合國跨政府氣候變遷研究小組(Intergovernmental Panel on Climate Change, IPCC)之第一工作小組於 2007 年提出的報告摘要，藉著報告中的科學證據說明為何要由人類負起責任著手降低大氣中的二氧化碳濃度。而在 IPCC 建議的三大減碳方向中，側重介紹與本篇論文研究較為相關的碳捕捉與封存(carbon capture and storage, CCS)技術，說明三個主要的碳捕捉途徑與實際施行捕碳時將會面臨到的困難，點出現階段碳捕捉材料的缺點以及為何要研發新一代的碳捕捉材料。而本篇論文研究的案例屬於金屬有機框架材料一類，正是下一代碳捕捉材料的候選者之一，因此接續簡介這類材料的特性、碳捕捉應用潛力以及目前用於提升這類材料碳捕捉性能的策略。並隨後說明本篇論文案例所屬的策略歸類，提出文獻上目前尚未進行探討的部分，帶出研究動機。最後於本章末詳細介紹本篇論文的研究案例。. 1.

(22) 1-1 溫室效應(Greenhouse effect) 溫室效應在談全球暖化(global warming)與為什麼要捕集與封存二氧化碳 (carbon capture and storage, CCS)之前，須要了解什麼是溫室效應 (greenhouse effect)。當太陽輻射能來到地球時，大約 1/3 會被地球大氣頂層直接反射回太空，剩下的 2/3 則少部分被大氣層吸收，絕大部分被地球表面吸收。被地球表面吸收的太陽輻射能，部份會以紅外線能量形式再釋放到大氣層中，甚至回到太空。這些釋放到大氣層中的紅外線，被大氣層中的溫室氣體 (greenhouse gas, GHG)分子與雲吸收後，會再次輻射回地球，這樣的情形可讓地球表面溫度維持在一定範圍，而這就是溫室效應。若是沒有地球本身自然發生的溫室效應的話，那麼地球平均溫度將會低於冰點，不利於許多生命存活。然而，由於人類過度使用化石燃料與砍伐森林造成屬於溫室氣體之一的二氧化碳在大氣中的濃度升高，溫室效應與日俱增導致了全球暖化[1]。. 1-2 全球暖化及相關全球暖化及相關科學證據及相關科學證據全球暖化(global warming)會導致海水溫度提高，讓海水體積變大以及大陸冰川與凍土等加速消融、崩解，海平面因此上升，淹沒沿海地區，使陸地面積縮小、土壤與地下水鹽化，影響許多生物存活[2]。此外，全球暖化將使熱浪、大火、乾旱與威力更為強大的颶風與颱風 2.

(23) 等極端氣候發生頻率與次數提升[2]。可說是牽一髮而動全身。上段提到許多因為氣候暖化所帶來的影響。20 世紀半葉以來的全球暖化現象真是人為因素所造成的嗎？持續暖化將會有什麼後果？該怎麼減緩全球暖化與氣候變遷的速率呢？這些問題的回應在聯合國跨政府氣候變遷研究小組(Intergovernmental Panel on Climate Change, IPCC)於 2007 年所發表的第四次全面性評估報告中都有說明 [1][3]. 。在 IPCC 第一工作小組(負責研究氣候變遷的科學機制)於該年 1. 至 2 月發表的報告中指出，過去半世紀以來的全球暖化，因人類活動使溫室氣體濃度升高讓溫室效應加劇的可能性高達 90 %[1][4]。研究人員發現溫室氣體濃度在過去 200 年來急劇暴增，而與工業革命前相較之下，現今大氣中二氧化碳濃度約高出 35 %、甲烷含量則為當時的 250 %、一氧化碳約多了 20 %。而且最近 10 年(從 2005 年算起)內二氧化碳濃度提升的速率比以往任何一個 10 年都還要快，而這些現象大多可歸因於人類活動[2][4]。這是因為化石燃料(fossil fuel，如石油、天然氣與煤)燃燒、農業活動等人類活動使得上述溫室氣體濃度(含人類合成的鹵烴)變高，這個結果能以同位素分析進行觀察而得知。此外，北半球陸地(人口較為密集之地區)之上的溫室氣體濃度較其他區域高[2][4]，亦顯現人類活動有其影響。 3.

(24) 研究人員以輻射驅動力(radiative forcing, RF)將各種影響全球氣候變熱轉涼的因素量化(見 Figure 1-1)。輻射驅動力係指相較於工業革命之前的時期至今，對流層頂的淨輻射通量的改變量，可表示地球整體能量平衡的變化。正向輻射驅動力會使氣候暖化，而負向輻射驅動力則相反[2][4]。從同一張圖可知正向輻射驅動力贏過負向輻射驅動力許多，這表示目前全球氣候偏向暖化。也能看出自然因素的輻射驅動力明顯小於人為因素造成的輻射驅動力。所有人為因素中由溫室氣體造成的影響比其他人為因素項目大上不少。而二氧化碳引起的正向輻射驅動力比其他溫室氣體的數值總合還來得高[2][4]。這就是要進行「減碳」的其中一項原因。. 4.

(25) Figure 1-1. Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). The net anthropogenic radiative forcing and its range are also shown. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness.[1][4]. 5.

(26) 而全球地表平均溫度升高、全球海平面上升以及北半球覆雪率下降等三大觀測結果也都證實了全球氣候正在暖化，而且暖化速率於近年攀升(Figure 1-2.)。研究人員將全球尺度與大陸尺度下的平均地表溫度觀測值與以氣候模型模擬考慮不同輻射驅動力時所得的數值進行比較，結果亦發現近半百年來的全球暖化可歸因於人為因素(見 Figure 1-3.)。當研究人員只將自然輻射驅動力(淡藍)納進考慮時，在近 50 年以來的部分，模擬結果與觀測值(黑色粗線條)不符，而當自然輻射驅動力與人為輻射驅動力(粉紅)同時採納時，模擬結果就與整體觀測結果相當吻合。研究人員也發現陸地上的暖化情況比海洋嚴重，而且淺層海水的暖化程度高於深層海水，人為因素造成的影響亦在此顯露蹤跡[2][4]。除造成暖化，大氣中的二氧化碳濃度增加，還會使得海洋的化學性質產生變化，例如海洋酸化。許多利用碳酸鈣建構外殼、其他堅硬部位(骨骼、耳石)的海洋生物，像是珊瑚、浮游生物與有殼生物將因海洋酸化使得碳酸鈣較難生成的緣故，降低其生存能力[5]，進而破壞海洋中的食物鏈與生態系，對整個生態圈都會產生影響。在 IPCC 評估報告中，關於情境模擬的部分有指出未來長期的氣候暖化幅度與快慢，將視溫室氣體減少排放的程度而定。機會稍縱即逝，在情況無法挽救前，須立即行動。減排方案其中一項就是下節將要介紹的碳捕捉與封存技術(carbon capture and storage, CCS)。 6.

(27) Figure 1-2. Observed changes in (a) global average surface temperature, (b) global average sea level from tide gauge (blue) and satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for the period 1961–1990. Smoothed curves represent decadal average values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c).[1][4] 7.

(28) Figure 1-3. Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings. Decadal averages of observations are shown for the period 1906 to 2005 (black line) plotted against the centre of the decade and relative to the corresponding average for 1901–1950. Lines are dashed where spatial coverage is less than 50%. Blue shaded bands show the 5–95% range for 19 simulations from five climate models using only the natural forcings due to solar activity and volcanoes. Red shaded bands show the 5–95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. [1][4]. 8.

(29) 1-3 碳捕捉與封存(carbon capture and storage, CCS) 碳捕捉與封存溫室氣體的主要來源是燃燒化石燃料後所排放出的二氧化碳，排放量為總溫室氣體排放量的 56.6 % (Figure 1-4.)[3]。而全球二氧化碳排放量已比過去(1970 至 2004 年間)增長了 80 %。並將會隨著世界人口數上升、經濟與工業發展而水漲船高。根本的原因無他，就是能源需求。以目前的趨勢而言，全世界對能源的需求將越來越高。據美國能源資訊署(Energy Information Administration, EIA)的預測，到了 2035 年，全球能源需求量將比 2008 年時增加 53%。從能源供給面分析，全球 85%的能源來源為石油、天然氣與煤炭等三種化石燃料[6]。在再生能源尚未能替代傳統能源之前，電力來源仍然會以火力發電為主，而火力發電廠的二氧化碳排放量占總排碳量的 30 到 40 %。若從這點進一步思考，在主要的排碳源進行碳捕捉與封存(carbon capture and storage, CCS)，電廠的年排碳量將有可能於短時間內大幅減少。因為電廠所產出的二氧化碳大部分都來自特定的操作單元，而且這些單元中的設備位置較為固定，只要在排碳源頭安裝碳捕捉系統便能有效率地控制二氧化碳排放量。. 9.

(30) Figure 1-4. Global greenhouse gas emission sources in 2004 of which approximately 77% are represented by CO2 emissions.[3][7] 而碳捕捉與封存技術技術主要是指將化石燃料轉為能源過程中主要是指將化石燃料轉為能源過程中產出的二氧化碳以相關碳捕捉碳捕捉技術將二氧化碳自排放廢氣分離技術將二氧化碳自排放廢氣分離出來，接著經壓縮後輸送至特定地點封存輸送至特定地點封存，以達成二氧化碳減量目標的技術達成二氧化碳減量目標的技術。整個過程包含二氧化碳捕捉二氧化碳捕捉、運輸和封存等三大環節[8]。本本篇論文主要簡介二氧化碳捕捉環節二氧化碳捕捉環節。運輸與封存環節將不多作說明。。而目前這項技術的研發重點在於提升碳捕捉效率研發重點在於提升碳捕捉效率、擴大捕捉規模以及降低捕捉成擴大捕捉規模以及降低捕捉成本。在能源生產過程中捕捉二氧化碳的途徑在能源生產過程中捕捉二氧化碳的途徑可根據發電廠將發電廠將燃料轉換熱或電能的方式進行進行分類，分別為燃燒後端捕捉途徑 (post-combustion)、燃燒前燃燒前端捕捉途徑(pre-combustion)與富氧燃燒與富氧燃燒後 10.

(31) 端捕捉途徑(oxy-fuel) fuel)等三種(Figure 1-5.)[7]。以下將為文個個別簡述各途徑的特點以及實施過程中在過程中在碳捕捉方面會遭遇的挑戰。在這之後將緊在這之後將緊接簡介現今正在發展的現今正在發展的碳捕捉技術以及碳捕捉材料，說明技術特性說明技術特性、碳捕捉材料種類以及研發方向以及研發方向。. Figure 1-5. Three options for CO2 capture from power generation plants. [7]. 11.

(32) 1-3-1 燃燒後端燃燒後端捕捉途徑捕捉途徑燃燒後端捕捉途徑中，碳捕捉是於火力發電廠燃煤或天然氣產生電力後所產生的廢氣(flue gas)進入氣體分離單元時進行。廢氣排放的氣壓約為 1 bar，氣體進入二氧化碳篩濾裝置的溫度範圍介於 40 至 60 ℃[9]。從這個途徑捕碳其中有一項優勢就是可直接對現有的電廠機組進行改裝，相關技術導入速率較快。不過二氧化碳在廢氣中所占的比例較低，燃煤發電廠的二氧化碳排放濃度為 15 至 16 %，而且廢氣的氣體組成較為複雜，包含氮氣(濃度為 73 至 77 %)、硫氧化物、氮氧化物以及其他氣體(Table 1-1.)[10]，加上各氣體分子之間的動力學直徑(kinetic diameter)差距很小[11]，這些狀況會使得碳捕捉難度較高，因此對碳捕捉材料有許多要求，例如高度的二氧化碳選擇性、二氧化碳吸附量大、熱穩定性佳與二氧化碳容易從碳捕捉材料中脫附而便於再生使用(regeneration)等[7]。燃燒後端捕捉途徑現階段所用的捕碳技術是以醇胺化學吸收法 (amine-based absorption)為主[12]，其原理是仰賴醇胺能與二氧化碳反應形成胺基酸酯或碳酸銨類的能力達成碳捕捉(如 Scheme 1-1.中的反應式)[13]。此方式的碳捕捉能力佳以及二氧化碳選擇性高，吸附熱可高達 50 至 100 kJ/mol[14]，但會使發電成本提高 30 %[15]。因為於再生 12.

(33) 時，電廠必須將一部分的能源產出用於加熱溶有醇胺與碳酸銨類的溶液，讓二氧化碳從溶液中釋出。而且此類吸收劑的高比熱以及過強的捕捉能力，這兩點都不利於再生使用。吸收劑也會隨著多次操作而耗損，並同時存在著腐蝕設備而提高維護成本的問題。因此降低再生時造成的能源耗損為目前新一代碳捕捉材料的研發方向之一，目標是將耗損率降至 10 至 20 %[16]。 Table 1-1. Typical Post-combusion Flue Gas Composition for a Coal-Fired Power Plant.[10] molecule. concentration (by volume). N2 CO2 H2O O2 SO2 SO3 NOx HCl CO hydrocarbons Hg. 73–77% 15–16% 5–7% 3–4% 800 ppm 10 ppm 500 ppm 100 ppm 20 ppm 10 ppm 1 ppb. 13.

(34) Scheme 1-1. Reaction of CO2 with Monoethanolamine (MEA) To Give a Carbamate mate Product (Upper), and the Corresponding Reaction with Triethanolamine (TEA) Resulting in a Bicarbonate Species (Lower). (Lower) [7][13]. 1-3-2 燃燒前端燃燒前端捕捉途徑捕捉途徑另一個碳捕捉的途徑是燃燒前另一個碳捕捉的途徑是燃燒前端捕捉。此途徑是先將先將化石燃料於氣室中不完全燃燒與氣化氣室中不完全燃燒與氣化產生合成氣(syngas) ，氣體大部分是由大部分是由氫氣、水氣、二氧化碳與一氧化碳二氧化碳與一氧化碳等氣體所構成(Table 1-2.)[17]。接著將合成。氣經過水煤氣轉化反應反應(water–gas shift reaction)變為水煤轉化轉化合成氣 (shifted syngas)，其主要成份為氫氣與二氧化碳。這時的溫度為溫度為 40℃，氣壓範圍在 5 至 40 bar 之間[7]。氣體中的二氧化碳的濃度氣體中的二氧化碳的濃度(35.5 %)明顯高過在燃燒後端捕捉途徑時的含量捕捉途徑時的含量(15 至 16 %)[18]，而且氣體組成而且氣體組成較為單純，有利於碳捕捉有利於碳捕捉，亦是優點之一。在經過氣體分離裝置將兩經過氣體分離裝置將兩者分離之後，氫氣作為發電用燃料氫氣作為發電用燃料，而二氧化碳則可進行壓縮與封而二氧化碳則可進行壓縮與封存。. 14.

(35) Table 1-2. Composition of Shifted Product of Coal Gasification.[17] component. approximate percentage (vol %). CO2 CO H2 CH4 H2O H2S N2 Ar. 25–35 0.5–0.7 30–50 0 15–40 0.1–0.2 0.3–2.3 0.04. 經由燃燒前端捕捉途徑進行捕碳還有兩個其他途徑所沒有的優點。其一，吸附著二氧化碳的吸附劑可直接透過氣壓變化(降壓)即可完成再生。其二，相較於燃燒後端捕碳(二氧化碳/氮氣分離)與富氧燃燒後端(氧氣/氮氣分離)捕捉途徑，在此途徑中要將二氧化碳與氫氣分開的難度較低，因為這兩個分子的分子量、電子雲極化率 (polarizability)與四極距(quadrupole moment)差異大(Table 1-3.)[19]。只要善用此差異，便可輕易地將兩者分離。. 15.

(36) Table 1-3. Physical Parameters of Gases Relevant to Carbon Dioxide Capture Processes.[19] molecule. kinetic diameter (Å). polarizability dipole moment (10–25 cm–3) (10–19 esu–1 cm–1). quadrupole moment (10–27 esu–1 cm–1). H2 N2 O2. 2.89 3.64 3.46. 8.04 17.4 15.8. 0.00 0.00 0.00. 6.62 15.2 3.9. CO NO H2O H2S. 3.76 3.49 2.65 3.60. 19.5 17.0 14.5 37.8. 1.10 1.59 18.5 9.78. 25.0. CO2 NO2. 3.30. 29.1 30.2. 0.00 0.00. 43.0. 1-3-3 富氧燃燒後端捕捉途徑富氧燃燒後端捕捉途徑第三種碳捕捉途徑為富氧燃燒後端捕捉。相較於燃燒後端捕捉途徑，此途徑的特色是讓發電用燃料在氧氣濃度較高的環境(氧氣與二氧化碳混合而成的氣體)下燃燒，因此排放出的廢氣經過冷凝分離程序(去除水蒸氣)之後，剩下氣體幾乎全是二氧化碳，所以碳捕捉將變得比較容易進行。整個過程中，會先將乾燥空氣中的氧氣分離出來並加以純化，接著會以二氧化碳(來自廢氣)將純度高達 95%的氧氣稀釋至 21%，以便控制燃燒時的溫度以及抑制氮氧化物和其他雜質的生成 [20]. 。而這個途徑所面臨的其中一項挑戰在於如何有效率地分離氧氣. 與氮氣。由於空氣中氧氣含量低，而且氧氣和氮氣分子的分子量、動 16.

(37) 力學直徑、電子雲極化率相近，以及四極距差異不大(Table 1-3.)[19]，因此要大規模地將氧氣從空氣中分離出來實屬不易。 1-3-4 碳捕捉技術與碳捕捉材料碳捕捉技術與碳捕捉材料二氧化碳捕捉技術包含溶液吸收法(solvent absorption)、低溫冷凝分離法(cryogenic carbon capture)、固態吸附劑捕捉法(solid sorbent)、薄膜分離法(membrane gas separation)等四種(Figure 1-6.)[21]。前兩種方法現今已有商業化產品。後三種方法目前正在發展當中。而這些方法中除了低溫冷凝分離法之外，都會需要碳捕捉材料吸收、吸附與分離二氧化碳(Figure 1-7.)[22]。溶液吸收法可據溶液吸收二氧化碳與從溶液中分離二氧化碳的方式大致分成兩類，一種是控制壓力與溫度改變吸收液中二氧化碳溶解量的物理吸收法。另一種為利用吸收液中的物質與二氧化碳發生酸鹼中和反應進行碳捕捉的化學吸收法[13]，而在簡介燃燒前端捕捉途徑中談到的醇胺化學吸收法即屬此類。此法常用的溶液種類有醇胺溶液[12]、氫氧化鈉溶液、氫氧化銨溶液、氟化物溶液與離子液體(ionic liquids, ILs)[23]等。低溫冷凝分離法則是藉冷凝原理分離二氧化碳與其他氣體。不過此法僅適用於混合氣體中某個氣體濃度偏高的情況，而且能源消耗量較大。可應用在富氧燃燒後捕捉途徑，生產途徑裡燃燒時所需要的氧氣。. 17.

(38) Figure 1-6. Carbon capture technologies include solvents, solid sorbents, membranes, and cryogenic distillation. [24]. Figure 1-7. Different carbon capture technologies and associated materials for CO2 separation and capture.[22] 固態吸附劑捕捉法是跨政府氣候變遷研究小組認為具有良好捕碳潛力而且未來將能投入實際應用的碳捕捉技術之一。此法以固態孔 18.

(39) 洞材料(solid porous adsorbent materials)作為吸附劑，利用其孔洞性質與吸脫附程序進行二氧化碳捕集或分離。使用固態吸附劑的好處有吸附劑的熱容較低、吸脫附速率較快、脫附時所需的能量較低以及裝置尺寸較小等。而吸附機制可依照吸附質(adsorbate)與吸附劑(adsortent) 之間的交互作用方式大致分為化學吸附(chemical adsorption)與物理吸附(physical adsorption)，兩者之間最大的不同在於有無發生化學反應，不過並沒有絕對清楚的分別，兩者間的差異可見 Table 1-4.。在實際應用場合中，此法常以雙或多塔操作的方式交錯進行吸附與脫附。操作型式主要有變溫吸附程序(temperature swing adsorption, TSA)、變壓吸附程序(pressure swing adsorption, PSA)與真空變壓吸附程序 (vacuum swing adsorption, VSA)等三種(Figure 1-8.)，其概略原理是藉溫度變化(TSA)或是壓力變化(PSA 和 VSA)捕捉與釋放吸附質。無論是何種程序都希望盡可能地提升有效容量(working capacity)以及降低再生時所需要的能量，使碳捕捉成本降到最低[25]。此法中吸附劑類型可依照操作時的溫度分類，有高溫吸附劑與低溫吸附劑兩種。在碳捕捉方面，前者以氧化鈣(calcium oxide, CaO)為主，後者是具有多孔性與高比表面積的材料。例如沸石(zeolites)[26]以及改質後的活性碳 (activated carbon)[27]。除了這兩種之外，還有其他具有碳捕捉潛力的材料，像是金屬有機框架材料(metal−organic frameworks, MOFs)[7][28]， 19.

(40) 本篇論文探討的案例就屬此類，將會於下一節簡介這類材料以及此材料在碳捕捉上的應用。上述碳捕捉技術中較合成本效益的技術有化學吸收法、固態吸附劑捕捉法與薄膜分離法，這三種方法的優缺點與應用途徑整理於 Table 1-5.。 Table 1-4. The differences between chemical adsorption and physical adsorption. Differences. Chemical adsorption. Physical adsorption. Forces. chemical bond formation. intermolecular interactions. Heat of adsorption. 40~4000 kJ/mol. ＜50 kJ/mol. Activation energy. Yes. No. Rate of adsorption. slower. faster. Reversibility. irreversible. reversible. Selectivity. higher. lower. Figure 1-8. Schematic diagrams of idealized temperature swing adsorption (TSA), pressure swing adsorption (PSA), and vacuum swing adsorption (VSA) processes for regenerating solid adsorbent in a fixed-bed column. [7] 20.

(41) Table 1-5. 化學吸收法、吸附法與薄膜分離法優缺點比較與應用途徑 [21]. 碳捕捉技術. 優點. 缺點. 應用途徑. ‧吸收劑成本較低. ‧須先處理硫氧化物. 燃燒前/後端. ‧處理容量大. ‧吸收液具腐蝕性. 捕捉途徑. ‧可分離出高濃度 CO2. ‧吸收液耗損問題. ‧吸收劑能再生使用. ‧碳捕捉時的耗能較高. 化學吸收法. ‧可分離出極高濃度 CO2 ‧分離效率受溫度影響燃燒前端、 ‧複合吸收法可提高 CO2 ‧須先加壓廢氣才可進燃燒後端與薄膜分離法. 與吸收劑間的氣液傳輸. 行分離. 富氧燃燒後. 與吸收速率. ‧薄膜製造成本較高. 端捕捉途徑. ‧較化學吸收法節省能源 ‧須先處理硫氧化物. 燃燒後端與. ‧可分離出高濃度 CO2. 富氧燃燒後. ‧反應器尺寸較小. 吸附法. ‧吸附劑劣化問題. ‧吸附劑可再生使用. 端捕捉途徑. 21.

(42) 1-4 金屬有機框架材料金屬有機框架材料(Metal− −Organic frameworks, 材料 MOFs) 金屬有機框架材料(metal−organic frameworks, MOFs)或可稱多孔性配位聚合物(porous coordination polymers, PCPs)[29]是近 20 年來引起許多研究團隊關注的新型孔洞材料。這類材料包含了以金屬為構成基底(金屬離子或金屬團簇)的節點(metallic nodes or connector)與橋接這些節點的有機配位子(organic ligand or linker)，由這兩者彼此自我組裝形成具有孔洞的三維網狀結構(Figure 1-9.)。兩者之間因配位數 (coordination numbers)與配位構形(coordination geometries)的多樣性而存在著許多組合(Figure 1-10.)[30]，不同的金屬離子搭配不同的有機配位子可配位形成性能迥異的結構，所以在與傳統的孔洞材料(如沸石與活性炭等)相較之下，金屬有機框架材料同樣可有高比表面積 (1500 至 7000 m2/g)與低密度(0.17 至 1.7 g/cm-3)之外[31]，此類材料還可擁有高度結晶性與伴隨此性質而來的明確孔洞性質，以及多樣的空間拓撲型態，其結構、孔洞和化學性質具備可設計性與可調變性等特質[32][33]。金屬有機框架材料的結構可藉拓展二級建構單元(連接點的延伸方式，secondary building units, SBUs)進行設計(Figure 1-11.與 1-12.)[34]，並且可用拓撲學(topology)探討這類材料的結構與連結點的連結模式(Figure 1-13.與 1-14.)[35]。 22.

(43) Figure 1-9. A portion of the crystal structure of Zn4O(BDC)3 (MOF-5). Blue tetrahedra represent ZnO4 units, while gray and red spheres represent C and O atoms, respectively; H atoms are omitted for clarity.[36]. Figure 1-10. Components of coordination polymers.[30]. 23.

(44) Figure 1-11. Different types of second building units (SBUs).[34]. Figure 1-12. Examples of SBUs from carboxylate MOFs. Color scheme: O, red; N, green; C, black. In inorganic units, metal-oxygen polyhedra are blue, and the polygon or polyhedron defined by carboxylate carbon atoms (SBUs) are red. In organic SBUs, the polygons or polyhedrons to which linkers (all –C6H4– units in these examples) are attached are shown in green.[34] 24.

(45) Figure 1-13. (a) Twelve of the binodal edge-transitive nets in augmented form. (b) Twelve more binodal edge-transitive nets in augmeted form.[35]. Figure 1-14. (a) the edge-transitive 2-periodic nets. (b) the augmented version.[35]. 25.

(46) 目前已有許多文獻報導金屬有機框架材料在許多領域上具備應用潛力，例如催化(catalysis)、感測(sensing)、藥物投遞(drug delivery)、分離純化(purification)以及氣體封存與分離等[37]。其中，在氣體吸附與分離方面，自從 Yaghi 主持的研究團隊率先利用金屬有機框架材料吸附二氧化碳之後(Figure 1-15.與 1-16.)[38]，這類材料的二氧化碳吸附性能開始受到關注，因為這類材料可以擁有高的二氧化碳吸附容量，而且具備可設計性與可調變性等特質，所以之後很有可能出現更適合讓二氧化碳附著其中的新型金屬有機框架材料，成為下一代碳捕捉材料的候選之一。而如何有效地提升材料的碳捕捉性能，為此領域其中一個重要的研發方向，下一節將簡介目前已發展出來的策略。. 26.

(47) Figure 1-15. Crystal structures of MOFs examined for CO2 storage capacity at room temperature. For each MOF, the framework formula, pore size, and surface area are given.[38]. Figure 1-16. Comparison of gravimetric CO2 capacities for several MOFs (and an activated carbon as a reference) determined at ambient temperature and pressures up to 42 bar.[38]. 27.

(48) 1-5 提升材料提升材料吸附二氧化碳能力的策略材料吸附二氧化碳能力的策略就如簡介碳捕捉途徑時所述，實際上要從這些途徑捕捉二氧化碳將面臨許多困境，像是廢氣的組成較為複雜、二氧化碳分子大小與其他氣體分子相近等。因此如何得以設計出具有高二氧化碳吸附容量與吸附選擇性的金屬有機框架材料一直是個相當有挑戰性的難題。近年在以金屬有機框架材料應用於碳捕捉的研究領域裡，研究人員不斷地致力於找出能夠提升材料的碳捕捉能力的方法。目前發展出的策略包含讓材料帶有極性官能基的吸附位置(chemical functionalization)[40]、引入陰/陽離子至材料當中(ionic frameworks)[41]、在材料中參雜金屬 (metal doping)[41]、框架互穿(interpenetration or catenation)[42]、建構尺寸較小或具特殊形狀的孔洞[43]以及讓材料含有配位未飽和之金屬位點(unsaturated metal coordination sites or open metal sites or exposed metals)[44]等(Figure 1-17.)。其中被廣為研究的策略為讓材料帶有氨基 (amine group)、含氮或其他高極性官能基的吸附位置與配位未飽和之金屬位點等兩種。在提升金屬有機框架材料應用於燃燒後捕捉途徑 (分離二氧化碳與氮氣)的碳捕捉性能方面，這兩種策略常被納入考量。由於燃燒後捕捉途徑目前已被採用，加上本篇論文將會討論到含醯胺官能基之金屬有機框架材料的 CO2/N2 分離性能，因此接下來將側重簡介這兩種策略。 28.

(49) Figure 1-17. Bottom-up Bottom strategies to tune CO2 capture performance.[22] The representative MOFs are from references[39]. references 1-5-1 材料含有配位未飽和之金屬位點材料含有配位未飽和之金屬位點(exposed 配位未飽和之金屬位點(exposed metals or open metal sites) 提升金屬有機框架材料之二氧化碳吸附選擇性的方式中提升金屬有機框架材料之二氧化碳吸附選擇性的方式中，其中一項就是讓材料含有配位未飽和之金屬位點含有配位未飽和之金屬位點(unsaturated unsaturated metal coordination sites or open metal sites or exposed metals)。此類吸附位點此類吸附位點的形成方式為合成材料時合成材料時，材料中的金屬或金屬離子除了與有機配位除了與有機配位子配位結合之外，還有溶劑分子還有溶劑分子與之配位，當材料經真空或高溫使溶當材料經真空或高溫使溶劑分子脫除後，金屬將呈現配位金屬將呈現配位未飽和(coordinatively-unsaturated unsaturated)的 29.

(50) 狀態，成為具有良好吸附能力的位點。擁有這類吸附位點的金屬有機框架材料，在低壓時可具有很高的二氧化碳吸附量。例如，M2(dobdc) (或稱 MOF-74、CPO-74，M = Zn, Mg, Co, or Ni)系列在 1 大氣壓與 296 K 時的二氧化碳吸附量可高達 24.4 至 35.2 wt% (8.00 mmol/g， MOF-74-Mg)。同溫度下，在 0.1 大氣壓時的二氧化碳吸附量高達 5.8 至 23.6 wt% (MOF-74-Mg)。這是因為 M2(dobdc)裡，呈六角柱狀之一維孔道的孔壁上，有著高密度的配位未飽和金屬位點，可讓二氧化碳緊附其上(Figure 1-18.)。正因如此，讓 MOF-74-Mg 於零覆蓋率時的二氧化碳等量吸附熱可高達 47 kJ/mol[45]，以及有著非常高的 15/85 CO2/N2 吸附選擇性，溫度於 296 K，從低壓至室壓的吸附選擇性數值為 412 至 196[46]。這裡提到的等量吸附熱(isosteric heat of adsorption, Qst)以及吸附選擇性(adsorption selectivity, Sads)，將會在下一章中介紹，這兩個數值是評估金屬有機框架材料碳捕捉能力的關鍵參數。另外，最早採用此策略的 Cu3(BTC)2 (香港科技大學 1 號，HKUST-1)[47]，由於有著配位未飽和的 Cu(Ⅱ)離子，因此其零覆蓋率時的二氧化碳等量吸附熱可達 29 kJ/mol。而 MIL(materials of institut Lavoisier)系列中的 MIL-100 與 MIL-101[48]在脫除水分子之後，Cu(Ⅲ)離子因此裸露，讓這兩個材料於零覆蓋率時的二氧化碳等量吸附熱分別能高達 62 kJ/mol 與 44 kJ/mol。然而廢氣中一些相對少量的氣體分子，像是水 30.

(51) 氣、硫氧化物與氮氧化物等，與配位未飽和之金屬位點之間也具有非常好的結合能力。因此這些分子會與二氧化碳一同競爭此類位點，並且非常有機會佔據位點，發生金屬毒化(poison)現象[49]。. Figure 1-18. (a) View of 1D channels present in the M/DOBDC structure (solvent omitted). (b) Space-filling model of the pore structure of 1 unit cell of Mg/DOBDC (Mg atoms, green; C atoms, gray; O atoms, red; H atoms and solvent molecules removed for clarity) occupied by 12 molecules of CO2 (blue) which represents the sorption at 0.1 atm and 296 K.[45] 1-5-2 將高極性官能基團引入材料之中(chemical functionalization) 將高極性官能基團引入材料之中相較於廢氣中的其他氣體分子，二氧化碳擁有很高的四極距與較 31.

(52) 大的電子雲極化率[19]。因此可活用此點，將高極性官能基團引入至金屬有機框架材料的孔洞當中，靠著那些對二氧化碳有著較高親和力的極性官能基團，把二氧化碳從混合氣體當中分離出來。引進的方式大致可分為兩種，一種是用於合成金屬有機框架材料的有機配位子本身就帶有含氮的極性官能基(pre-synthetic functionalization)，本篇論文探討的案例─含醯胺官能基之金屬有機框架材料就屬此類。而極性官能基當中，具鹼性的含氮官能基最常被考慮引進材料當中。例如， IRMOF (isoreticular metal organic framework)系列中，使用帶有氨基的 2-aminoterephthalic acid (NH2-BDC)作為有機配位子的 IRMOF-3(Zn4O(NH2-BDC)3)，在 298 K 與 1.1 bar 時的二氧化碳吸附量(5.0 wt%)高於其母系化合物─IRMOF-1(或稱 MOF-5，Zn4O(BDC)3) (4.6 wt%)[50]。另外，Vaidhyanathan 等人以 3-amino-1,2,4-triazole (Atz) 為關鍵配位子進行合成的 Zn2(Atz)2(ox)[51]，在 273 K 與 1.2 bar 時，其二氧化碳吸附量遠高於氮氣吸附量，在零覆蓋率時的二氧化碳等量吸附熱高達 40.8 kJ/mol (Figure 1-19.)。另一種引進極性官能基的方法是將現有的金屬有機框架材料進行化學修飾(post-synthetic modification)。例如，Long 等人將乙二胺(ethylenediamine, en)或 N,N-二甲基乙烯基二胺(N,N'-dimethylethylenediamine，mmen)嫁接至 CuBTTri 的配位未飽和金屬位點，合成出 en-CuBTTri[52]與 mmen-CuBTTri (Figure 1-20.) 32.

(53) [53]. 。發現 en-CuBTTri 在 298 K 與低壓(0.06 bar)時的二氧化碳吸附量. (0.366 mmol/g, 1.6 wt%)優於未經化學修飾的 CuBTTri (0.277 mmol/g, 0.92 wt%)。而 mmen-CuBTTri 在 298 K 與低壓(0.15 bar)的二氧化碳吸附量(2.38 mmol/g, 9.5wt%)是 CuBTTri 的三倍。而且同樣的溫度與壓力條件下，mmen-CuBTTri 的 CO2/N2 吸附選擇性數值(327)可突破 300，並擁有極高(近 100 kJ/mol)的二氧化碳吸附熱(於零覆蓋率時的數值)。後來，此研究團隊將 N,N-二甲基乙烯基二胺嫁接至 Mg2(dobpdc)的配位未飽和金屬位點得到 mmen-Mg2(dobpdc)，此化合物在仿一般空氣條件(298 K，二氧化碳分壓為 0.39 mbar)時的二氧化碳吸附能力相當突出(2.0 mmol/g, 8.1 wt%)，而在仿燃燒後捕捉途徑的廢氣條件(313 K，二氧化碳分壓為 0.15 bar)下的二氧化碳吸附容量高達 3.14mmol/g (12.1 wt%)。因此發現這種做法能夠有效提升材料在低壓時的二氧化碳吸附量[54]。然而，提高材料胺化的程度(higher degrees of amination) 就一定有利於捕捉二氧化碳嗎？其實並不盡然，當材料中的胺基型吸附位置分布密度太高與過於緊密時，這些胺基反而會妨礙彼此與二氧化碳結合的能力[55]。. 33.

(54) Figure 1-19. (Left) (a) Structure of the Zn-Atz layer in Zn2(Atz)2(ox). (b) Three-dimensional structure of Zn2(Atz)2(ox), wherein the Zn-Atz layers are pillared by oxalate moieties to form a six-connected cubic network shown as green struts. (c) Adsorption isotherms for different gases carried out using Zn2(Atz)2(ox). (Right) X-ray structure of CO2 binding in Zn2(Atz)2(ox)·(CO2)1.3 at 173 K. (a) The role of the amine group of Atz in binding CO2-I is depicted. The H atoms of the amine group (located crystallographically) H-bond to oxalate O atoms, directing the N lone pair toward the C(δ+) atom of the CO2 molecule. H-bond distances shown are for H-acceptor interactions. (b) Both crystallographically independent CO2 molecules are shown trapped in a pore, showing the cooperative interaction between CO2-I and CO2-II molecules. The CO2…NH2 interaction is represented as a dotted purple bond, and the CO2…CO2 interaction is indicated as a dotted yellow bond. (c) This panel shows the other interactions present. The CO2-I…Ox interactions are shown in orange, and the CO2…NH2 hydrogen bond interactions are shown in green. For clarity, H atoms are shown in purple.[51] 34.

(55) Figure 1-20. (a) Functionalization of the metal−organic framework Cu-BTTri through binding N,N'-dimethylethylenediamine (mmen) to the open metal coordination sites. The basic amine groups dangling within the framework pores lead to strong CO2 adsorption, with a zero-coverage isosteric heat of adsorption of −96 kJ/mol. (b) CO2 (■) and N2 (●) adsorption isotherms collected at 298 K for mmen-Cu-BTTri (green symbols) and Cu-BTTri (blue symbols). (c) a plot of the isosteric heat of adsorption of CO2 (■) and N2 (●) in mmen-Cu-BTTri (green symbols) and Cu-BTTri (blue symbols).[53]. 35.

(56) 除了上述胺基官能基之外，若改以其他類型的極性官能基引入金屬有機框架材料之中，同樣也有機會提升材料的二氧化碳捕捉能力[56]。例如，Banerjee 等人研究發現 8 個具有相同結構特徵(拓撲皆為 GME) 的類沸石結構之金屬咪唑框架材料(zeolitic imidazolate frameworks, ZIFs)之中，就屬有著硝基的 ZIF-78 具備最高的二氧化碳吸附選擇性以及二氧化碳吸附容量[57]。而含有氧原子的羥基亦可提升材料的二氧化碳吸附選擇性。像是 Yonggang Zhao 等人在研究 Zn(bdc-OH)(ted)0.5 的二氧化碳捕捉性質時，觀察到此化合物的孔容和表面積雖然因為孔洞中多出羥基的關係，都比母系化合物(Zn(bdc)(ted)0.5)小，不過有著羥基的 Zn(bdc-OH)(ted)0.5 的二氧化碳吸附量比較高(bdc=terephthalic acid, ted=triethylenediamine, bdc-OH=2-hydroxylterephthalic acid)[58]。而 Sihai Yang 等人讓孔洞中含有羥基的金屬有機框架材料(NOTT-300) 經過粉末 X-ray 繞射分析(power X-ray diffraction)與非彈性中子散射 (inelastic neutron scattering)分析之後，觀察到此材料可靠著羥基捕捉二氧化碳(-O-H(δ+)…(δ-)O=C=O)，並且因為有著適當的孔洞環境以及二氧化碳等量吸附熱，因此可在不損及吸附容量的情況下，降低再生時所需要的能量，提升材料的二氧化碳捕捉效率[59]。. 36.

(57) 而本篇論文案例所採用的醯胺官能基(amide group)也是值得納入考量的選項。近年，Zaworotko 與其他共同研究人員經比較兩個準結構同晶體(pseudo-structural isomorphs)之金屬有機框架材料 ([Cu24(TPBTM6−)8(H2O)24]與 PCN-61)的二氧化碳吸附性能後，觀察到孔洞中含有醯胺官能基的[Cu24(TPBTM6−)8(H2O)24]，其二氧化碳吸附量與等量吸附熱都比較高，認為醯胺官能基與二氧化碳之間的偶極四極交互作用與氫鍵(N-H…O=C=O)可以提升材料的二氧化碳吸附性能(Figure 1-21.)[60a]。不過此例以及之後衍生出的相關系列都含有配位未飽和的金屬位點[60]，並非單靠醯胺官能基作為吸附位點。而其他引入醯胺官能基的案例[61]由於缺乏更多實驗或理論計算結果的幫助，因此無法進一步釐清醯胺官能基是如何影響著材料的二氧化碳吸附能力。而且以上這些案例，其醯胺官能基在孔洞中的分佈較為稀疏，一般都認為這些官能基都是獨自與二氧化碳交互作用，彼此之間少有合作或競爭。另外，目前未有文獻討論醯胺官能基的空間佈置對於材料吸附二氧化碳的影響，此點正為本篇論文的探討重點。而論文裡的兩個案例，其醯胺官能基在孔洞中的空間佈置情形與上述案例截然不同，能夠營造出獨特的孔洞環境，且未包含配位未飽和之金屬位點等其他類型的吸附位置。關於這兩個含醯胺官能基之金屬有機框架材料的合成方法、晶體結構與孔洞環境等將於下一節簡介。 37.

(58) Figure 1-21. (a) N,N',N''-tris(isophthalyl)-1,3,5-benzenetricarboxamide (TPBTM) and 5,5′,5′′- benzene-1,3,5-triyltris(1-ethynyl-2-isophthalate) (btei). (b) Portion of the structure of the (3, 24)-connected rht-type framework of [Cu24(TPBTM6-)8(H2O)24](1)showing surface decoration by acylamide groups. (c) High-pressure gravimetric excess CO2 and N2 sorption isotherms of [Cu24(TPBTM6-)8(H2O)24](1)and the PCN-6X series at 298 K. Lines and symbols represent adsorption and desorption, respectively. (d) Isosteric heats of CO2 adsorption for [Cu24(TPBTM6-)8(H2O)24](1) and PCN-61.[60a]. 38.

(59) 1-6 介紹研究介紹研究案例研究案例─ 案例─化合物 1 與化合物 2 1-6-1 化合物合成簡介本篇論文探討了兩個含醯胺官能基之金屬有機框架材料的二氧化碳捕捉性質，在接下來的內容當中接下來的內容當中分別以化合物 1 與化合物 2 稱之。化合物 1 是利用 ZnCl2、1,4-benzyl benzyl dicarboxylic acid (H2bdc)與 (H2bdc N,N′-bis(4-pyridinyl)-1,4 1,4-benzenedicarboxamide (bpda)在 DMF/H2O 溶液中以溶劑熱法(120 ℃，72 小時)經一步自我組裝步驟合成經一步自我組裝步驟合成而得 (Scheme 1-2.)。而化合物化合物 2 同樣採用溶劑熱法，以 CdCl2、 2,6-naphthalenedicarboxylic naphthalenedicarboxylic acid (2,6-H (2,6 2ndc) 以及 1,3,5-benzene benzene tricarboxylic acid tris[N-(4-pyridyl)amide] tris[N (4-btapa) 在 H2O/DMF 溶液裡於 120 ℃並經 72 小時的反應，透過一步自我組裝合成步驟得到小時的反應組裝合成步驟得到 (Scheme 1-3.)。 Scheme 1-2.. Synthesis of compound 1 by a one-step step self-assembly self process.. 39.

(60) Scheme 1-3. Synthesis of compound 2 by a one-step self-assembly process.. 1-6-2 化合物結構描述 {[Zn4(bdc)4(bpda)4]·5DMF·3H2O}n (化合物 1) 單晶繞射分析(single-crystal X-ray diffraction analysis)顯示化合物 1 的晶系為單斜晶系(monoclinic system)，空間群為 P21/c [62]。這個非對稱單元包含了 4 個 Zn(Ⅱ)離子、4 個 bdc2-有機配位陰離子、4 個 bpda 中性有機配位子、5 個二甲基甲醯胺(Dimethylformamide, DMF)以及 3 個水分子。構成化合物 1 框架的二級建構單元(secondary building unit, SBU)是由雙金屬核心{Zn2N4O6}團簇節點被 4 個 bdc2-有機配位陰離子與兩個由 bpda 中性有機配位子成對相疊構成的柱狀建構單元以八面體的方式連結而成，具有 6 個連接點，而外觀類似二葉輪槳(Figure 1-22 a 與 b)。化合物 1 中，每個 bdc2-有機配位陰離子以 µ3-架橋配位的方式連結 3 個 Zn(Ⅱ)離子，在這當中，配位子上其中 1 個羧酸官能基以 µ2-η1:η1-架橋配位模式與 Zn(Ⅱ)離子相連結，另一個羧酸官能基 40.

(61) 則採取單芽配位(Figure 1-22 b)。化合物 1 整體的三維網狀結構為具 412·63-pcu 拓撲的網狀框架兩兩互穿的結果(Figure 1-22 c)。結構中， bpda 有機配位子裡的其中一個醯胺官能基能與 bdc2-有機配位子的羧酸官能基形成 N-H…O 氫鍵(N…O = 2.89 Å)，而這股存在於網狀結構之間的氫鍵能夠幫助穩固化合物 1 的整體結構(Figure 1-23 a)。沿著 b 軸俯視化合物 1 的晶體結構，可見到兩個尺寸大小不同(2 × 3 Å2 與 3 × 9 Å2)的一維貫穿孔道(Figure 1-23 b)。當脫除所有的溶劑分子後，主孔道(尺寸較大)立壁裡，彼此櫛比鱗次般正反相疊的醯胺官能基會因此顯露而不受遮蔽(Figure 1-23 c)。如此獨特的官能基空間佈置對於化合物 1 的二氧化碳捕捉性質會帶來什麼影響，此為本篇論文欲深入探討的部分。. Figure 1-22. Structures of 1: (a) Side view of an α-Polonium-type cage. (b) Simplified view of a six-connected node. (c) The two-fold interpenetrated 412·63 net. 41.

(62) Figure 1-23. (a) Net-to-net N−H…O hydrogen bonding interactions of in 1. (b) Superimposed space-filling representation of 1 showing two different sized channel. (c) A spotlight of the larger channel opening showing a nearly unique arrangement of the unsheltered amide-groups in 1.. 42.